Detection of Miocene saucer‐shaped sills (offshore Senegal) via integrated interpretation of seismic,magnetic and gravity data

The Atlantic margin offshore Senegal has been explored by seismic reflection and GRAV‐MAG surveys. High‐amplitude, laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs. Once seismic data are integrated with potential fields modelling, these reflectors can be safely interpreted as saucer‐shaped igneous sills, up to some hundreds of metres thick, some km wide. The occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the Miocene. Field observations and in‐situ magnetic susceptibility measurements of Oligocene–Miocene and Quaternary igneous rocks emplaced in coastal Senegal support this interpretation.     

Comparison of surface wind stress characteristics over the Tropical Atlantic (10°N–40°S) in fields derived from the UWM/COADS, NCEP/NCAR and QuikSCAT datasets

A comparison of monthly wind stress derived from winds of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis and UWM/COADS (The University of Wisconsin-Milwaukee/Comprehensive Ocean-Atmosphere Data Set) dataset (1950–1993), and of NCEP/NCAR reanalysis and satellite-based QuikSCAT dataset (2000–2006), is made over the South Atlantic (10°N–40°S). On a mean seasonal scale, the comparison shows that these three wind stress datasets have qualitatively similar patterns. Quantitatively, in general, from about the equator to 20°S in the mid-Atlantic the wind stress values are stronger in NCEP/NCAR data than those in UWM/COADS data. On the other hand, in the Intertropical Convergence Zone (ITCZ) area the wind stress values in NCEP/NCAR data are slightly weaker than those in UWM/COADS data. In the South Atlantic, between 20° S–40°S, the QuikSCAT dataset presents complex circulation structures which are not present in NCEP/NCAR and UWM/COADS data. The wind stress is used in a numerical ocean model to simulate ocean currents, which are compared to a drifting-buoy observed climatology. The modeled South Equatorial Current agrees better with observations between March–May and June–August. Between December–February, the South Equatorial Current from UWM/COADS and QuikSCAT experiments is stronger and more developed than that from NCEP/NCAR experiment. The Brazil Current, in turn, is better represented in the QuikSCAT experiment. Comparison of the annual migration of ITCZ at 20° and 30°W in UWM/COADS and NCEP/NCAR data sources show that the southernmost position of ITCZ at 30°W in February, March and April coincides with the rainy season in NE Brazil, while the northernmost position of ITCZ at 20°W in August coincides with the maximum rainfall of Northwest Africa.     

Compaction and cover effects on runoff and erosion in post-fire salvage logged areas in the Valley Fire,California

Runoff and erosion processes can increase after wildfire and post-fire salvage logging, but little is known about the specific effects of soil compaction and surface cover after post-fire salvage logging activities on these processes. We carried out rainfall simulations after a high-severity wildfire and post-fire salvage logging to assess the effect of compaction (uncompacted or compacted by skid traffic during post-fire salvage logging) and surface cover (bare or covered with logging slash). Runoff after 71 mm of rainfall across two 30-min simulations was similar for the bare plots regardless of the compaction status (mean 33 mm). In comparison, runoff in the slash-covered plots averaged only 22 mm. Rainsplash in the downslope direction averaged 30 g for the bare plots across compaction levels and decreased significantly by 70% on the slash-covered plots. Sediment yield totalled 460 and 818 g m⁻² for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by an average rate of 71%. Our results showed that soil erosion was still high two years after the high severity burning and the effect of soil compaction nearly doubled soil erosion via nonsignificant increases in runoff and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor controlling runoff, while surface cover was the dominant factor for rainsplash and sediment yield. Saturated hydraulic conductivity and interrill erodibility calculated from these rainfall simulations confirmed previous laboratory research and will support hydrologic and erosion modelling efforts related to wildfire and post-fire salvage logging. Covering the soil with slash mitigated runoff and significantly reduced soil erosion, demonstrating the potential of this practise to reduce sediment yield and soil degradation from burned and logged areas.     

The Risk Atlas of Mexico City,Mexico: a tool for decision-making and disaster prevention

Natural Hazards - We present a Risk Atlas of Mexico City based on a Geographical Information System (RA-GIS). We identified the prevalent social risk to the more relevant hazards in Mexico City...     

On the swimming strategies to escape a rip current: a mathematical approach

Rip currents represent significant hazards to swimmers all around the world. The danger arises when a misinformed swimmer uses an inadequate strategy to escape the rip, such as fighting the current directly. This can lead to fatigue, panic, and in some cases drowning. There exists a range of strategies put forward by experts (both lifeguards and scientists) to escape rip currents. However, these recommendations are based on a limited number of scientific studies and there is still much discrepancy surrounding the best strategy to escape a rip. Thus, here we present a simple, physics-based theoretical model aimed at assessing different escape strategies in terms of their associated ‘energetic cost’ (in work and power) for any given rip current and swimmer’s proficiency level. Many combinations of swimmers and rips are considered, including both idealised and a realistic rip current. Our quantitative results back the common knowledge that swimming against the rip (which is strongly discouraged by lifeguards) is almost universally the worst possible strategy, especially when compared against strategies favoured by experts, such as floating with the current before attempting to swim back to the shore. For a realistic rip, our results suggest that swimming directly against the rip can require several times more power from the swimmer than other strategies advised by lifeguards, thus lending further scientific support to experts’ recommendations. This study may help promote education around the dangers posed by rip currents and how best to address them.

     

Transport and dispersion of particulate Hg associated with a river plume in coastal Northern Adriatic environments

The role of suspended particulate matter (SPM) as an important carrier of mercury (Hg) dispersed into the Gulf of Trieste and in the adjacent Grado lagoon (Northern Adriatic Sea) was studied during a high Isonzo River inflow and the resulting river plume formation. Despite the fact that extreme flood events are rare during the year, they account for most of the PHg influx (37-112 ngL(-1)) into the Gulf of Trieste. When the river plume is diverted to the SW under the influence of an E-NE wind, the tidal flux acts as a "transport belt" carrying the PHg, mostly inorganic, into the Grado lagoon. A preliminary estimation indicates that the amount of PHg entrapped in the lagoon basin following a tidal semi-cycle accounts for 1.4 kg/12h, which corresponds to about 49% of the total Hg carried by the tidal flow. These findings should be considered in future remediation strategies in the lagoon environment.     

Time-dependent degradation of biotic carbonates and the search for past life on Mars

Searching for traces of extinct and/or extant life on the surface of Mars is one of the major objectives for remote-sensing and in-situ exploration of the planet. In the present paper we study the infrared (IR) spectral modifications induced by thermal processing on differently preserved calcium carbonate fossils, in order to discriminate them from their abiotic counterparts.The main conclusion of this study is that the degree of alteration of the fossils, derived from IR spectral analysis, seems to be well correlated with the sample age, and that terrestrial fossils after a billion years are so altered that it becomes impossible to trace their biotic origin. Since it is reasonable to assume that the putative Martian fossils should be at least 3.5 billion years old, this would imply that our spectroscopic method could not be able to detect them, if their degradation rate were the same as that we have found in usual conditions for the terrestrial fossils. However, due to the different climate evolution of the two planets, there is the possibility of having two different degradation rates, much lower for Mars than for Earth, especially if the fossils are embedded in a protective layer, such as a clay deposit. In this case IR spectroscopy, coupled with thermal processing, can be a useful tool for discriminating between abiotic and biotic (fossil) carbonate samples collected on the Martian surface.     

Temperature trends in Libya over the second half of the 20th century

This paper investigates spatial variability of temperature trends over Libya in the second half of the 20th century. The study is based on complete and homogeneous time series of minimum, maximum, and mean temperature for ten observatories. During the investigated period (1951–1999), temperature trend analyses have experienced a downward trend in the maximum surface temperature (about –0.06°C decade^–1) and an upward trend in the minimum surface temperature (about 0.23°C decade^–1). Cooling tendency in maximum temperature is spatially more pronounced in inland stations compared to coastal stations. At the seasonal scale, maximum temperature cooling is more obvious in winter and spring, meanwhile minimum temperature warming is more pronounced in summer and fall. In accordance with global trends, the surface mean temperature has moderately risen at an average rate of 0.09°C decade^–1. However, this trend has shown considerable temporal variability considering a more pronounced upward trend in summer and fall. In conjunction with other regional and global investigations, clear trends towards smaller diurnal range are presented (–0.28°C decade^–1).     

New data for the kinematic interpretation of the Alps–Apennines junction (Northwestern Italy)

Alps and Apennines are juxtaposed within an approximately 100 km-wide area covered by the Upper Eocene to Miocene successions of the Tertiary Piedmont Basin. The Upper Eocene–Oligocene evolution of this area was characterized to the north and west by the propagation of the SE-verging Southalpine thrust-fold belt that can be traced from the Po Plain subsurface until the Torino Hill-Saluzzese area, and to the south by a high-angle, broadly E–W oriented megashear zone that led to the juxtaposition of different crustal levels and controlled the development of a mosaic of partly independent sub-basins. Since the latest Oligocene the N-verging Apenninic tectonics prevailed in the collisional system and the Tertiary Piedmont Basin evolved as a wide thrust-top basin, bounded to the north by the N-verging Monferrato arc and characterized by a tectono-sedimentary evolution recording changes of subsidence and shift of depocentres in relation to crustal structures.     

Landslides induced by the April 2007 Aysén Fjord earthquake, Chilean Patagonia

On the 21st of April 2007, the Aysén Fjord earthquake (Mw 6.2) in southern Chile (45.3° S, 73.0° W) triggered hundreds of landslides in the epicentral area along the fjord coast and surroundings. Some of these landslides induced large tsunami waves within the fjord causing fatalities and damaging several salmon farms, the most important economic activity of the area. The landslides included rock slides and avalanches, rock falls, shallow soil and soil–rock slides, and debris flows. The earthquake was the climax of a seismic swarm that began 3 months earlier. The seismicity is associated with tectonic activity along the Liquiñe–Ofqui fault zone (LOFZ), a major structural feature of the region. The earthquake-induced landslides were mapped and classified from field observations and remote sensing analysis. The landslide areas and epicentral distances are within the expected range for the earthquake magnitude according to worldwide data, while the position of landslides on the slopes strongly suggests topographic amplification effects in triggering the failures. The location of the landslides is also clearly related to some of the main fault branches of the LOFZ. The seismic event has configured a new situation of seismic and landslide hazard in the Aysén region and along the LOFZ, where the presence of towns and economic infrastructure along the coasts of several fjords constitutes a potential risk that was not considered before this seismic event.